Device for determining birefringence

ABSTRACT

A DEVICE FOR ELECTRICALLY AND DIRECTLY INDICATING THE BIREFRINGENCE OF A TRANSPARENT MATERIAL WHICH UTILIZES AN ELECTRO OPTICAL SENSOR HAVING A COMBINATION OF POLARIZERS, BIREFRINGENT PLATES AND PHOTOCELLS TO PRODUCE VOLTAGES FUNCTIONALLY DEPENDENT UPON THE BIREFRINGENCE OF THE TRANSPARENT MATERIAL. DIFFERENCE AMPLIFIERS AND A CATHODE RAY TUBE OR SERVO SYSTEM ANALYZE THE PHOTOCELL PRODUCED VOLTAGES TO GIVE A DIRECT INDICATION OF THE BIREFRINGENCE OF THE TEST MATERIAL.

Filed May 3, 1968 2 Sheets-Sheet l 3 Fig. 4b

' INVENTORS Harold Levensfein Albert Goldstein Audrey J. Hebling ATT EY Jan. 19, 1971 2 Sheets-Sheet 2 Filed May 5, 1968 ited States its. Cl. 356-114 8 Claims I ABSTRACT OF THE DISCLOSURE EA device for electrically and directly indicating the b'refringence of a transparent-material which utilizes an :ectro optical sensor having a combination of polarizers, refringe nt plates and photocells to produce voltages fpnctionally dependent upon the birefringence of the transparent material. Difference amplifiers and a cathode ray tr be or servo system analyze the photocell produced voltages to. give a direct indication of the birefringence of the test material.

BACKGROUND OF THE INVENTION This invention relates generally to devices for determining birefringence, and more particularly to a device which electronicallydetermines directly the birefringence of a transparent material.

Alarge varietyof optically transparent materials exhibit the phenomenon of birefringence or the splitting of at ray of incident light into two components which may tfiavel at different velocities. In such materials the dielectiic constant, e, is a function of the direction in which it is measured. For example, the relationship between the electric displacement vector, D, and the electric field intensity vector, E, for a given material can be given by a real symmetric tensorrelationship in terms of the components-ofeach vector expressed in a convenient Cartesian system as follows:

1=11 1+ 12 z+ ia a a= 21 1+ 22 2+ aa a a= s1 1+a2 2+ as 3 When a-suitably oriented set of axes are chosen the aforesaid equations will simplify to:

Dp= E1 D2=2E2 D3=3E3 When the above dielectric constants are identical the material is optically isotropic and no birefringence will r -r sult. However, when the .dielectric constants are different,

i the material will be optically anisotropic and the phenomenon of birefringence will occur. When birefringence takes place, the mode of propagation of the light within the material will depend upon the direction of propagation of the entering light.

The birefringence of-transparent materials may be usefully. employed in many optical devices. For example, bi-

'refringence may be used to produce multiple images, for

optical filtering and in alignment mechanisms. In many materials birefringence occurs because of internal strains and may be used therefore as an indication of an undesirable state as for example-in-improperly annealed glass and regions in plastic assemblies around holes and in- ,serts. Birefringence may be used in structural analysis of atent crossed polariz'ers. Nicol prisms or Senarmont prisms. Such prior art techniques have not been entirely satisfactory in thatthey require the use of moving pptical elements and therefore are incapable 'of responding to 5 rapidly changing'dynamic stress conditions in a test? medium. Thus, the t-espouse of such prior art techniques is limited to frequencies below the megacyc'le range/Another disadvantage of the prior art is its inflexibility with respect to change of optical source frequencies. Systems used in the prior art have reference plates which mist be precisely a quarter wave of the optical source freqii'e'ncy if an accurate measurement is to be made. No techii'ique is present in the," prior art to compensate for the offnominality of these quarter wave plates when the optical source frequency .is changed.

SUMlii ARY OF THE INVENTION Accordingly, one object of this invention is to prgvide a new and improved device for determining birefringence. Another object of the invention .is the provision fof a new and improved device for directly determining the birefringence of a'transparent material.

Still another object of the present invention is togprovide a new and improved device for directly and electrically determining the birefringence of a transparent material.

A still further object of the instant invention the provision of a new and improved device for determining birefringence which is capable of responding to rapidly 3O changing dynanjiii': stress conditions. One other object of this invention is to provide a; ,new and improved device for determining birefringence which is capable of use with a wide range of optical source frequencies. I h

Briefly, in acordance with this invention, these and other objects are attained by directing a polarized beain of light upon a material under test whose birefringence: is to be determined. The light exiting from the material optically analyzed and then converted into electrical signals 40 which when analyzed give a direct measurement of the birefringence of the test material. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention "and many of the attendant advantages thereof will be readily appreciated as .the same becomes better understood by reference to the'following detailed description when considered in connection with the accompanying drawing wherein:

FIG. 1 is a perspective view of the electro optical sensor component of the birefringence determining device of the instant invention; FIG. '2 is across-sectional view of the electro optical sensor shown in FIG. 1; p v

FIG. --3 isa block diagram of the preferred embodiment of the birefringence determining device of the present invention; p Y a FIGS. 4a, 4b and 4c are alternative electro optical sensor components for use in thesubject invention; and

FIG. 5 is another embodiment of the birefringence determining device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings wherein like reference characters designate identicalor corresponding .parts throughout the several views and more particularly to FIGS. 1 and .2 thereof wherein a preferred embodiment of the" basic electro optical sensor component of the device to be hereinafter described for determining birefringence is shown as consisting essentially of a conventional light source 10, such for example as a standard Patented Jan. 19, 1971 monochromator or laser, a conventional collimator 12, such for example as a convergent lens, conventional polarizers 14 and 22-25, such for example as Polaroid sheets, a material 16 whose birefringence is to be determined and materials 18 and 20 whose birefringence is known. As shown in FIG. 2, conventional detectors 26-29, such for example as photocells, are a part of the electro optical sensor.

In operation, parallel light emitted from source through collimating lens 12 passes through a linear polarizer 14; whose axis of polarization is oriented at 45 to the X axis. Only, that light polarized with respect to the predetermined axis is supplied to the material 16 under test. The birefringence that will be measured will be that due to the difference in-the index of refraction along the X and Z directions. The beam of light exiting from the material 16 is divided into four separate light beams either by physically dividing the area of light or by theuse of optical mirrors as shown in FIGS. 4b and 4c and more fully explained hereinafter. The first beam of light will pass through the polarizing analyzer 22 which is orieiited so that its axis of polarizationis parallel to the axis of polarization of polarizer 14. Accordingly, the

transmission of light through the polarizer 14, material 16, and analyzer 22 will be proportional to T which depends upon birefringence in the following manner:

where is the relative retardation or birefringence of the material. The second beam of light will pass through the polarizing analyzer 23 which is oriented so that its axis. of polarization is perpendicular to the axis of polarization of polarizer 14. The transmission of light that passes through analyzer 23 is proportional to T in 35 the following manner:

where a is theknown birefringence of the crystal material'18. The fourth beam of light passes through the crystal material20'whose known birefringence is subtracted from the :birefringence of the material 16"under test and th'enpasses through analyzer 25. To effect the subtractionof the birefringence of the crystal material 20 "from that of material 16, the orientation of crystal material 20 is set so that its fast axis F is rotated 90 from the fast axis Z of the material The transmission of light that passes through the analyzer 25 is then proportional to T in the following manner:

It should be understood that the crystal materials 18 and 20 are identical so that the known birefringence oz of both are equal. Normally, crystal materials 18 and 20 are chosen so that the birefringence a approximates 90. If a is exactly 90 then the orientation of crystal materials '18 and 20 relative to material 16 is unimportant, however. analyzers 24 and 25 must still lie at 45 to the respective axes of. crystal materials 1 8 and 20. Normally, the axis of polarization of analyzers 24 and 25 are parallel to the axis of polarization of polarizer 14.

If crystal materials 18 and 20 are chosen such that a=90+6, where 6 is a small amount, then the transmissions T through T become as follows:

In order to interpret these transmissions, each beam of light exiting from the analyzers 22-25 is supplied to linear response photodetectors 22-25 respectively, where electrical signals E through E, are generated in proportion to the amount of light received.

In the preferred embodiment for determining thebirefringence of the test material in FIG. 3, the electrical signals E through E are subtractedin pairs in conventional difference amplifying circuits 36 and 38 to produce the following outputs:

1= 1( 1 2) 1 cos I H2=K2(T3-T4)==K2 Sin ()5 COS 6 I v where K and K are constants. If the constant K; of difference circuit 38 is adjusted to equal K /cos 6, then the outputs H and H from the difference circuits will become:

as is varied, and the polar angle measured'from the horizontal'will give a direct electronic measurement of the birefringence 15 of the test material 16.

It shouldbe understood that the electro optic sensor I crystal arrangement described above has been chosen to allow operation with crystal materials (normally 2; wave plates) whose birefringence is not exactly This had been done deliberately since unless the chosen source frequency is exactly set to the frequency of the Mt wave plates, the birefringence will not be exactly 90"-. Accord? ingly, by using the optical arrangements described and the electrical compensating technique the sensor is made independent of the sourcefrequency and deviationsin birefringence of the /4 wave plates-and the system can operate Witha wide range of frequency source It should be further understood that whilea specific arrangement of cystals, or quarter-wave platesand polarizers for establishing'tlie desired four beams of light have been shown, other alternative arrangements are 65"- sible and useful in particular cases. I a l For example, in FIG. 4a the light exiting from material 16 strikes a=conventional non-dichroic beam splitting or partially reflective mirror 30. One-half'of the light will pass through beam splitter 30, while the other half will be reflected by the mirror. The use of such beam splitters allows for the use of a beam of smaller cross section, such as a laser, and also allows for a smaller cross set-ition of crystal material-to be used. Eacliof these two bear}n's of light maybe physically separated into two additional'-'* beams to enable the desired four beams of light to be obtained. It will be apparent as shown in FIG. 411' that two additional beam splitters 32 and 34 may be used to obtain the desired four beams without the need forany in the crjoss section of the material and; for smaller crossbe used. Additionally, FIG, illustrates section beams to a still fgrther arrangement wherein is shown the mbi+ nation polarizers 48,-a'quarter-wave plate 50, a halfwave .pl, e'52, a test material 54 and analyzers 56. this "combination, the plates must be calibrated at 903' and 180 respectively, so .asto achieve the desired four .b eams. It shd'uld 2 be understood that; while FIG. 3 shows a cathod ray tube'to indicate the birefringence of the .test material; that a servo system :such as that shown'in fFlG. S'm'ay lie used. Signals H and H from differe circuits'36 iand 38 arev fedto thestatb'l'gterminals-60 a ard '62 ofra coj ventional resolver 58. Atconventional feedback system ijvhich comprises a se'rvo amplifier 66, moto {68 andgea'r r70 allowsfor-a direct indication of th fringencta of the test material to be directly indicated on amechanical dial ordigitalindicator 72 via rotor Obvicjusly, numerous other modifications and .gvariations ofyithe present invention are'possible in light the above teachings. lttis therefore to be understo that within the'scopeof the appended claims the invention'may be practiced otherwise than as specifically described whati is-claimed as new and desir'e'd to besecuifefd by Letters Qatent oftheUnited States is: i l. A idevice for determining birefringence-comprising means for projectingpolarized light through a ihaterialgwhose birefringence is to be determined, means; for projecting a portion of the light eme ging from said materialawhose birefringence is to be idetermined through a material whose birefringence is known, first analyzer means for providing optical signals responsive to saidrlight as it emerges from said material whtise birefringence isto be determined, second analyzer means :for providing .optical sjgnals responsive ,to the portion of the light-projected through said material whose birefringence is kitown, means for -detecting said optical signals responsiYe to said light as it emerges from said material ijhose birefringence is to be determined and for'detecting said optical signals responsive to the portion oj' the light "projecting through said material whose fn'ngence is-knownand forgproviding electrica sig- 111318 responsive to said detected-optical signals, and meansresponsive to said electrical signals for providing an indication of, the birefringence of .saidmaterial whose'birefringence is toi-be determined. 2. A device for determining birefringence according to claim 1 wherein I said material whose birefringence is known first and, second materials whose birefringence is known, a I i said 'first analyzer means for providing optical signals includes 6 the light emerging from said ,meterial whose birefiin genee, ,-is to berdetermined j-and- Q a i said third and said fourth analyzing polarizers' are oriented to. provide optical signals, respectively, given by the relation: 7

wherein T is an optical signal proportional to the portionof the light projected through said first ma- I terial whose birefringence, a; is known and T is an optical signal proportional tdfthe portion of the light projected through said second material whose birefringence, ,is known audio is the birefringence -';of-"said*material whose birefringence is to be-determined. 4. A device for determining birefringence as in claim 3 wherein said means for detectgingsaid' optical signals includes four photocell detectors: each of which is resonsive to the optical .si'gnal' trans'gnited from a-respective one of saidfour-analyzing polarfiers whereby four electrical signals representative of said four optical signals are generated. *5. 'A device for determining birefringence as in claim 4 wherein said means responsive to saidelectrical' signals includes' two difference amplifiersibne of" wh'ich is 'responsive' to the two of said four elecitrical signals generated which are representative of said @two optical signals represented by "T and T and the other of which is responsive to the o'thertwo of said foiir electricalsignals gen- 'eratedwhichare representative 59f said two'additio'na'l' optical signals represented by T f'nd T whereby the outputs of the-two difference amplifiers respectively generate signals according to the following relationsz includesa cathode ray tube, the horizontal andvertical plates of which are respectively connected-to the. outputs (if-631d two difference amplifiers whereby the bire'frim jge'nce of saidmaterial whosebirefringence is 'torbe deter- :rnmed is {directly obtained. 5

inel'udes'first'and second analyzing polarizers re- J sponsive to the light as it emerges from said rn aterial-whose 'birefringe'nce is to be determined, and a said second analyzer means for providing optica1 Sigl nals includes third and fourth analyzing polari'zcrs responsive to the portion of the light projectedtlirough said firstand said second materials: whose'-=birefrin+.

gence isknown, respectively. '3. A *device fordetermining to-claimZ wherein said first and said second analyzing polarizers are oriented,to;. provide optical, signals, respectively, givenby the relation:

birefringence 1 according detectormeansiresponsive tq thertlight feom said sanv alyzingapolamer means forsproviding electrical i '7. A device for determining birefringence assin .claim l ivherein said meanstfor .projecting.,.p0larized light :includes v light source, acollimator and a polarizer.

*8. A device for determining'birefri gence comprising -means for-projecting polarized'light' aiong afirst and "a material whose birefringence-is to be determined, means for projecting-polarized light '-.'along a third, and fourth path, said third path and said' fourthv-paths including a material-whose.birefringence is known andsaid material whose birefringence is to be de- ;polarizer-i rieansf.rfor analyzing the light projected t-hrough'ithe birefringent materials along each ofi-said r: V hm ---m;1 s responsive-theretmfand .means esponsiv'ekto -said el i l-S gn ls provid- "m 2 ."l s c of.the birefringence .of' said material 9 i i ce is -to be determined.

'- (ReEerene es on f6l|0Willg-.,pa

I 1 a I fringence as in claim 5. wherein-said means for projecting polarized alight "includes,

esecqnd .path, ."sa id firstand-said se'condrpaths including 3,345,905 10/1967 'Aloque 356-116 .7 References Cited 7 1 ow Mpthod for Measurement of Birgfringence and'Di- UNITED STATES PATENTS chroism Rey; Sci. Instr., vol. 38, #5, May 1967,-pp. 625- 2,877,683 3/1959 Fischer 2s0 -225 Sr'1e 11, Towards Automatic Measurement of Birefrin- '5 gence," Nature,-vo1. 214, Apr. 1, 1967, pp. 78-79. FOREIGN PATENTS ,064 1958 Great Britain 356-416 AL L-WIBERT, i y a iner 1,002,616 1965 Great Britain 356114 J. ROTHENBERG, Assistant Examiner 1,313,997 1962 France 1 356116 OTHER REFERENCES us. c1. X.R.

Schmidt-Tiedemann, l mproved Electronic Half-Shad- 250225; 350-157; 356 -116 

